1. Field of Invention
The present invention relates a lens inspection apparatus and an inspection sheet.
2. Description of Related Art
Recently, projectors have been used which include a plurality of liquid crystal panels for modulating a plurality of colored light beams according to image information, a crossed dichroic prism for combining the colored light beams modulated by the corresponding liquid crystal panels, and a projection lens for enlarging and projecting the light beams combined by the prism so as to form a projection image. The optical characteristics, such as image resolution and chromatic aberration, of the projection lens used in the projectors sometimes vary, for example, depending on variations in the production processes. Since the variations in optical characteristics of the projection lens have an influence on the quality of the image displayed by the projector, the resolution of the lenses is evaluated before the lenses are shipped by the lens manufacturer and before the lenses are assembled into the projectors.
More specifically, a test pattern for resolution measurement is formed on an inspection sheet, image light containing the test pattern is introduced into a projection lens by irradiating the test pattern with light, and the image supplied from the projection lens is projected onto a screen. Then, the image of the test pattern projected on the screen is detected by an image capture apparatus using an image pickup device, such as a CCD (Charge Coupled Device), and the image detected by the apparatus is subjected to image processing by a computer or the like, thereby evaluating the resolution of the projection lens. Since the test pattern includes a plurality of small patterns of a predetermined size, the plurality of small patterns are projected on the screen. The image pickup device sequentially moves over the small pattern images projected on the screen, and captures the images at the corresponding positions.
Test patterns to be formed on known inspection sheets are quite small so that the outer size thereof is, for example, approximately 10.8 mmxc3x9714.4 mm. These test patterns can include a plurality of small patterns, as described above, and the outer size of the small patterns is even smaller, for example, 795 xcexcmxc3x971074 xcexcm. Since the test patterns of the inspection sheets have such a quite fine structure, they are manufactured in a process similar to a semiconductor manufacturing process. That is, first, chromium (Cr) is evaporated onto a glass substrate after a mask having a predetermined test pattern is produced beforehand. Then, a photoresist (photosensitive resin) is applied on the chromium on the glass substrate, and is irradiated with ultraviolet rays through the above mask. Subsequently, portions of the photoresist irradiated with the ultraviolet rays are removed with a predetermined solvent. In this way, the mask is transferred onto the photoresist. By then subjecting the chromium to etching with the photoresist used as a mask, a chromium test pattern is formed on the glass substrate.
A plurality of types of inspection sheets are necessary in accordance with the outer sizes of liquid crystal panels used in the projector. Since a plurality of types of inspection sheets are manufactured through such a process, the cost of the inspection sheets is increased. Since the resolutions of a plurality of small patterns formed in the test pattern are evaluated while moving the image pickup device to the image positions of the small patterns, as described above, it takes a long time to move and place the image pickup device at predetermined positions, and fast inspection is impossible.
Accordingly, an object of the present invention is to provide a lens inspection apparatus and an inspection sheet that make it possible to easily evaluate the resolution of a lens at low cost.
A lens inspection apparatus of the present invention inspects a lens in order to evaluate the resolution of the lens by projecting image light containing a test pattern for resolution measurement onto a screen through the lens, and displaying an image of the test pattern for resolution measurement on the screen. The lens inspection apparatus can include an inspection sheet having the test pattern for resolution measurement, an inspection-sheet holder for holding the inspection sheet, a light source for introducing a light beam to the test pattern of the inspection sheet held by the inspection-sheet holder, and an image-light detecting section having an image pickup device for capturing an image projected on the screen through the inspection sheet. The test pattern can include a measuring region in which linear light-shielding portions are arranged in stripes in order for transmitted light to have a predetermined spatial frequency, and the inspection-sheet holder includes a holder body having a light-transmissive section corresponding to the test pattern of the inspection sheet for positioning the inspection sheet at a focal position of the lens, and a rotary holding portion for holding the inspection sheet so that the inspection sheet rotates in a plane relative to the holder body.
As the image pickup device, image pickup devices, such as CCDs and MOS (Metal Oxide Semiconductor) devices, may be used.
The image-light detection section may include the above-described image pickup device, an image data generating device, such as a video capture board, for receiving an output from the image pickup device and for converting the output into image signals for a computer, and a computer for processing the image signals. The computer may include a program running under an OS (Operating System) for controlling the operation of the computer, and the program may include, for example, a program for evaluating the resolution, the chromatic aberration, and the like.
While the predetermined spatial frequency may be arbitrarily determined, for example, it can be set to be within the range of 20 lines per millimeter to 80 lines per millimeter.
In the lens inspection apparatus of the present invention, for example, when a light beam is emitted from the light source after the inspection sheet is held in the inspection-sheet holder so that the light-shielding portions of the test pattern arranged in stripes extend in the vertical direction (upward-downward direction), the light beam travels through the inspection sheet and the projection lens, and a test pattern image in which the light-shielding portions extend in the vertical direction is projected onto the screen. By capturing the projected image with the image pickup device, and then detecting the captured image with the image detecting section, the resolution of the projection lens can be evaluated.
Next, the rotary holding portion can be rotated in a plane relative to the holder body, and is fixed at a position such that the direction of the light-shielding portions is different from the above direction (vertical direction). For example, the rotary holding portion is rotated by 90xc2x0, and is fixed so that the light-shielding portions extend in the horizontal direction (rightward-leftward direction). In this state, the resolution of the projection lens can be evaluated in a manner similar to the above.
By changing the orientation of the test pattern by rotating the rotary holding portion in a plane after loading the inspection sheet having the test pattern into the inspection-sheet holder, as described above, the resolution of the projection lens can be easily evaluated while one type of test pattern, in which the light-shielding portions extend in different directions, for example, in two directions, that is, the vertical direction and the horizontal direction, is projected onto the screen. In this way, the number of types of inspection sheets to be prepared can be reduced, and the cost of the inspection sheets can be reduced. In this case, simply by rotating the rotary holding portion, the resolution evaluation can be easily switched to a resolution evaluation with an inspection sheet in which the light-shielding portions extend in a different direction, and this can shorten the inspection time. In other words, the inspection operation can be simplified.
Preferably, the inspection-sheet holder has a sliding holding portion for holding a plurality of inspection sheets having test patterns of different spatial frequencies, and for holding the inspection sheets so that the inspection sheets can slide in a plane relative to the holder body. In such a configuration, by sliding the sliding holding portion while it holds a plurality of inspection sheets having test patterns of different spatial frequencies, an inspection sheet having a desired spatial frequency can be selected from a plurality of inspection sheets. Furthermore, the extending direction of the light-shielding portions in each inspection sheet can be selected by rotating the rotary holding portion, as described above. For this reason, for example, when two types of inspection sheets (test patterns) are placed in the sliding holding portion, the resolutions of at least four types of test pattern can be evaluated. Therefore, the number of inspection sheets to be prepared can be reduced, and the cost of the inspection sheets can be further reduced. In this case, it is possible to easily switch between the types of inspection sheets and to shorten the inspection time with a relatively simple structure for sliding and placing the inspection sheet using the sliding holding portion. This can simplify the inspection operation.
When the resolution of a general-purpose projection lens, which is not required to have a higher level of precision than necessary, is evaluated, it is satisfactory to inspect four types of test patterns in which the measuring region extends in the horizontal and vertical directions and the spatial frequency of the measuring region differs. For this reason, when the sliding holding portion is provided with two holding frames for holding an inspection sheet, and these two holding frames are slid by the sliding holding portion and are rotated by the rotary holding portion, inspection can be easily performed by using all four types of test without exchanging the inspection sheets simply by first precisely loading two types of inspection sheets in the two holding frames, respectively, and then operating the rotary holding portion and the sliding holding portion. Therefore, it is possible to simplify the inspection operation and to shorten the inspection time.
Preferably, the image-light detecting section includes a plurality of image pickup devices for capturing an image projected on the screen, and the plurality of image pickup devices are fixed to the screen. In such a configuration, by adjusting the position of the inspection sheet beforehand so that an image of the test pattern is properly projected onto the positions where the fixed image pickup devices perform detection, the image pickup device does not need to be moved to the test pattern images projected on the screen, which was necessary before, and therefore, the inspection time can be shortened.
The chromatic aberration of the lens can also be evaluated with such an inspection sheet for resolution evaluation. In this case, in order to evaluate the chromatic aberration of the lens, it is preferable that the lens inspection apparatus of the present invention have the following structure. In other words, first, the lens inspection apparatus may have a filter mounting portion in which color filters for transmitting only a light beam having a wavelength within a predetermined range from among the light beams emitted from the light source are mounted.
As the combinations of colors of such color filters, for example, a combination of three primary colors, red (R), green (G), and blue (B), or a combination of three complementary colors, cyan (C), magenta (M), and yellow (Y) may be adopted. The three primary colors provide higher color reproducibility, and the three complementary colors provide higher resolution. These three colors may be appropriately changed depending on the application and so on. The color filters are not limited to the above combination of three colors, but may have four or more colors or two colors or less.
In such a configuration, the chromatic aberration of the lens is evaluated, for example, in the following procedure. That is, first, the above-described color filters of three colors are prepared and are loaded in the filter mounting portion. Then, one of the three color filters is selected, and is placed in the optical path of a light beam from the light source. In this state, the light beam is emitted from the light source to the color filter, and a light beam having a frequency within a predetermined range passing through the color filter passes through a predetermined test pattern, and a test pattern image is projected on the screen. Subsequently, the image projected on the screen is captured by the image pickup device, and the position of the test pattern in the captured image is stored in the image-light detecting section. Next, a color filter of another color is placed in the optical path, and the position of the test pattern is stored in a manner similar to the above. The position of the test pattern for the remaining color is similarly stored. The stored test pattern positions for the corresponding color filters are subjected to pattern matching, thereby evaluating the chromatic aberration.
Accordingly, the chromatic aberration of the lens can be easily evaluated simply by mounting the color filters in the filter mounting portion and alternatively placing these color filters in the optical path of the light beam from the light source. Since the filter mounting portion has a structure in which color filters, which are cheaper than the image pickup device, are simply placed in the optical path, it can be easily produced. Therefore, the filter mounting portion, as well as the color filters, can be cheaply produced, and the cost of evaluating the chromatic aberration of the lens can be reduced.
Second, in the lens inspection apparatus, the image-light detecting section may be provided with a prism for separating the image light projected on the screen into a plurality of colored light beams, and the image pickup device may be placed at each light-emergent end face of the prism corresponding to the colored light beams.
In such a configuration, the chromatic aberration of the lens is evaluated, for example, in the following procedure. That is, a light beam emitted from the light source passes through a predetermined test pattern without passing through the above-described color filters so as to project a test pattern image onto the screen. The projected test pattern image is separated into color light images by the prism. Subsequently, images corresponding to the separated colored light beams are substantially simultaneously captured by the image pickup devices, and the images corresponding to the colored light beams captured by the image pickup devices are stored and are subjected to pattern matching by the image-light detecting section, thereby evaluating the chromatic aberration of the lens.
Since the images corresponding to the colored light beams are simultaneously captured by the image pickup devices in this way, it is unnecessary to exchange the color filters, which is necessary in the above-described case using the color filters, and this can shorten the time taken to measure the chromatic aberration of the lens.
An inspection sheet of the present invention is a rectangular inspection sheet that has a test pattern for resolution measurement on the upper surface thereof and that is placed on the upstream side of a lens in the optical path so as to evaluate the resolution of the lens by projecting image light containing the test pattern for resolution measurement onto a screen through the lens, and displaying an image of the test pattern for resolution measurement on the screen. The inspection sheet has a measuring region in which a plurality of linear light-shielding portions are arranged in stripes between a pair of opposing edges.
In the present invention, since a plurality of linear light-shielding portions are formed in stripes as the measuring regions between a pair of opposing edges, by setting the outer size of the test pattern including these measuring regions to be, for example, equal to or more than the size of the largest liquid crystal panel to be used in projectors, a single inspection sheet can also function as a plurality of types of inspection sheets having different outer sizes, regardless of the outer size of the liquid crystal panel, and the cost of the inspection sheet can be reduced. In this case, since the inspection sheet can be used as a plurality of inspection sheets, it does not need to be replaced even when the outer size of the liquid crystal panel changes when replacing the lens to be inspected. Therefore, the inspection time can be easily shortened. In other words, the inspection operation can be simplified.
Preferably, a plurality of measuring regions are arranged in the extending direction of the pair of edges, and a light-transmissive region that does not have the light-shielding portions is formed between the adjoining measuring regions.
An MTF (Modulation Transfer Function) may be used as a resolution evaluation value for evaluating the resolution of the lens. When it is assumed that the maximum detected brightness of the test pattern image is Imax, the minimum brightness is Imin, and the brightness of a background portion in which the test pattern is not formed is Io, the MTF is found from:
MTF=(Imaxxe2x88x92Imin)/(Ioxc3x972xe2x88x92Imaxxe2x88x92Imin)xe2x80x83xe2x80x83[Equation 1]
When the MTF serving as the resolution evaluation value can be measured according to Equation 1, a proper MTF, which does not include the offset in the data generating device, can be found.
Therefore, in the above configuration, since the maximum and minimum brightnesses can be detected at the measuring regions, and the brightness of the background portion can be detected at the light-transmissive region, the MTF serving as the resolution evaluation value can be found from Equation 1. For this reason, the resolution of the projection lens can be more accurately evaluated without being influenced by the model of the projector, the position on the display image, and the like.